Beat the cream cheese, heavy cream and mustard in a medium bowl with an electric mixer until fluffy. Combine the salmon and onions in a small bowl.
Cut the cucumber crosswise into 1/3-inch rounds. Scoop 1/2 teaspoon cream cheese mixture onto each radish slice and place it on a piece of cucumber, cream cheese-side down, pressing lightly. Top with a small sprig of dill, followed by 1/2 teaspoon salmon mixture.
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